When there is no existing macvlan port in lowdev, one new macvlan port
would be created. But it doesn't be destoried when something failed later.
It casues some memleak.

Now add one flag to indicate if new macvlan port is created.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>